In recent years, the use of cellular communication systems having mobile communication units which communicate with a hardwired network, such as a local area network (LAN) and a wide area network (WAN), has become widespread. Retail stores and warehouse, for example, may use cellular communications systems to track inventory and replenish stock. The transportation industry may use such systems at large outdoor storage facilities to keep an accurate account of incoming and outgoing shipments. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects.
A typical cellular communication system includes a number of fixed base stations or access points interconnected by a system backbone. Such cellular communication systems which have a system backbone may also be hereinafter referred to as a network communication system. Also included in many cellular communication systems are intermediate base stations which are not directly connected to the system backbone. Intermediate base stations, often referred to as wireless base stations or repeaters, increase the area within which base stations connected to the system backbone can communicate with mobile communication units.
A cellular communication system includes several base stations, each of which transmits and receives RF signals throughout a predetermined coverage area referred to as a cell. A cell is a geographic area in which a base station has sufficient signal strength to transmit and receive data from a mobile communication unit or other device with an acceptable error rate. Thus, the outer boundary of a cell is determined not only by the radiated power of the base station transceiver (which determines the maximum range at which a mobile communication unit is able to receive a signal), but also by the effective radiated power of the mobile communication units (which determines the maximum range at which the base station is able to receive a signal). Typically, base stations will be positioned along the backbone such that the combined cell area coverage from each base station provides full coverage of a building or site. Further, it is also typical to have the cell area of coverage from two or more base stations to overlap or be colocated.
The shape of each cell is primarily determined by the type of antenna associated with a given base station. For instance, base stations which communicate with mobile communication units often have omnidirectional type antennas which provide for generally circular shaped cells and allow for a wide area of coverage. In many instances, however, the cell of a base station is not completely symmetrical because physical structures within the cell may partially block signals emanating from the base station or create "dead spots" where no signal can pass. Further, the cell size may be decreased by machinery located in the vicinity of the base station which generates excessive noise levels that degrade a signal transmitted by the base station.
Generally, when a mobile communication unit is powered up, it "registers" with a base station through which the mobile communication unit can maintain wireless communication with the network. In order to register, the mobile communication unit must be within the cell range of the base station and the base station must likewise be situated within the effective cell range of the mobile communication unit. It is generally not possible to have one base station service a large area by itself. This is due to transmission power restrictions governed by the FCC and the fact that such a large cell range would add significantly to the size and weight of the mobile communication unit thereby making it less desirable to use. Thus, cellular communication systems generally have several base stations spaced apart such that the collective cell area coverage of the base stations is sufficient to cover the entire area in which the mobile communication unit may roam. As the location of the mobile communication unit changes, the base station with which the mobile communication unit was originally registered may fall outside of the geographic cell range of the mobile communication unit. Therefore, the mobile communication unit may "deregister" with the base station it was originally registered to and register with another base station which is within its current communication range.
The mobility of the mobile communication unit in the aforementioned systems creates the need to determine the respective location of the mobile communication units within the network. Systems for locating mobile communication units are known, however, most of these systems employ global position system (GPS) technology. The Global Positioning System (GPS) is a network of approximately 24 satellites and a dozen ground stations. The GPS network currently provides navigation information worldwide. A receiver derives its three-dimensional position from ranging signals received from three or more satellites. However, GPS technology requires the use of expensive equipment and is generally not practicable for use in most local area network (LAN) based systems.
U.S. Pat. No. 5,519,760 ('760) to Borkowski et al. discloses a cellular position locating system which does not requires the use of GPS technology. However, the '760 patent only teaches a method for locating mobile devices in a cellular telephone network having a mobile switching center which provides necessary processing and switching to allow cellular phones to communicate with each other. This, of course, would have no benefit for locating mobile devices in a LAN, WAN or other cellular based system which does not have such a mobile switching center. Furthermore, the location information obtained in the system of the '760 patent only pertains to which of several cell sites a mobile unit is currently located. As cell site coverage areas are often very large, such information would not be sufficient for quickly and efficiently locating a mobile device.
In view of the aforementioned shortcomings associated with conventional communication systems involving locating mobile devices, there is a strong need in the art for a system and method for locating a mobile device in a cellular communication system wherein existing network components can be used to accurately narrow down and/or pinpoint the location of a mobile communication unit within the system.